It is known that in these cells the anode reagent is constituted by an alkaline metal, generally sodium, which must be liquid at operation temperature. The cathode reagent is generally constituted by sulphur and the sodium salts of this substance, but can also be constituted by phosphorus, selenium and the alkaline salts of these substances. In the case where the reagent materials are sulphur and sodium, the electrochemical reaction leads to the reversible formation of sodium polysulphides whose sodium content increases during discharge. As for the electrolyte, which separates the cathode and anode reagents, it must be solid at operation temperature, i.e. about 300.degree. C., permeable to the alkaline ions which form in the anode compartment and impermeable to electrons. It is generally constituted by beta sodium alumina, i.e. a compound having about 5 to 9 alumina molecules for one sodium oxide molecule. It is generally in the shape of a tube closed at its bottom, containing the anode reagent and immersed in the cathode reagent, the latter reagent being contained in a metal cathode tank and impregnating a graphite felt. The electrolyte tube is held by a support connected in a fluid-tight manner to this cathode tank and also to an anode reagent tank.
Generally, said support is in the form of a plate or disk made of alpha alumina. This disk has a central bore in which the beta sodium alumina is fixed.
In such cells, the discharge process leads to the transformation of sulphur into sodium polysulphuides in the sequence set forth hereinbelow: EQU S .fwdarw.Na.sub.2 S.sub.5 .fwdarw.Na.sub.2 S.sub.4 .fwdarw.Na.sub.2 S.sub.3
during recharging, the reverse process occurs. However, it is observed that it is practically impossible in conventional cells to effect the transformation: EQU Na.sub.2 S.sub.5 .fwdarw. S
and consequently to effect complete recharging.
This disadvantage appears to result from the fact that during recharging a film of sulphur is permanently deposited on the electrolyte tube which fulfills the function of a migration barrier with respect to the sodium ions and thus opposes the total regeneration of elementary sulphur.
On the basis of this experimental observation the applicant has produced a new structure for a sodium-sulphur cell suitable for remedying such a disadvantage and consequently for enabling a practically complete recharging of said cell.